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Award Ceremony Speech

Elementary particle physics which is now so vigorous was still in
its infancy when Murray Gell-Mann in 1953 published the first of
the papers which have been honoured with this years Nobel Prize
in physics.

The physicists were, however, already then acquainted with a
rather large number of particles which apparently were
indivisible and therefore elementary building stones of all
matter. The elementary particle known for the longest time was
the electron.

New particles were added when the atomic nuclei were explored. It
was found that the atomic nuclei consist of positively charged
protons and electrically neutral neutrons. These particles are
held together in the atomic nuclei by enormously strong forces
called nuclear forces which do not distinguish between protons
and neutrons. This symmetry of the nuclear forces was expressed
by saying that the nuclear forces are charge-independent. A
proton and a neutron have further very nearly the same mass. They
form a doublet of particles and have been given the common name
of nucleons.

An increase already expected and desired occurred in the family
of elementary particles at the end of the 1940's, when new
particles called pi-mesons were discovered. They were named
mesons because they have a mass between the electron and the
nucleon masses. The pi-mesons had been predicted by the Japanese
physicist Yukawa. They form a
triplet of particules having nearly the same mass but different
charges which are + 1, 0 and -1 in units of the proton charge.
Their interaction with the nucleons is strong and
charge-independent. Their most important task is to be an
intermediary agent for the strong interactions between the
nucleons.

A very remarkable discovery which marked a new area in particle
physics was made by the British physicists Rochester and Butler
about the same time. They found new unstable particles which did
not fit in with the theoretical ideas developed so far. Some
of the new particles are heavier than the nucleons and were
grouped together with them under the common name of baryons. The
others were lighter than the nucleons but heavier than the
electrons and were called K-mesons. The new particles were
copiously produced when high-energy pi-mesons collide with
nucleons and were therefore assumed to interact strongly with
other particles. But they had such a long lifetime that some law
must exist which prevent the strong forces to act when they
disintegrate into other particles. Gell-Mann discovered this law
after some preliminary results had been found by Pais.

It had been assumed earlier that the new baryons from doublets
like the nucleons and that the K-mesons form triplets like the
pi-mesons. Gell-Mann made the fundamental new assumption that the
new baryons instead form a singlet, a triplet and a doublet, the
latter being different from the nucleon doublet, and that the new
mesons form two kinds of doublets, one consisting of the
antiparticles of the other. Gell-Mann assumed further that the
principle of charge-independence was generally valid for strong
interactions. He could thereby explain the mysterious properties
of the new particles. He introduced a new fundamental
characteristic of a multiplet called its hypercharge. This is
defined as twice the mean value of the charges in the multiplet.
Gell-Mann's proposed the new rule: Elementary particles can be
transformed in others by the strong and the electromagnetic
interactions only if the total hypercharge is conserved. This
rule reminds of the law of conservation of the electric charge.
It should be remarked that Gell-Mann initially used instead of
the hypercharge a closely related number called the
strangeness.

This discovery by Gell-Mann was admirable considering in
particular the very meagre experimental material available to
him. In the predicted baryon multiplets there occurred empty
places. Gell-Mann could on this ground predict two new baryons.
One of them was soon discovered but the other not until six years
later.

This classification of the elementary particles and their
interaction discovered by Gell-Mann has turned out to applicable
to all strongly interacting particles found later and these are
practically all particles discovered after 1953. His discovery is
therefore fundamental in elementary particle physics.

It should be added that two Japanese physicists, Nakano and
Nishijima, published a similar classification some months later
than Gell-Mann.

Many theoretical physicists tried during the following years to
find new symmetries which should give relations between the
particle multiplets. Initiated by Sakata a series of papers were
published in particular by Japanese physicists. They indicated
that a certain kind of symmetry could be of interest. Gell-Mann
showed in a new fundamentally important paper of 1961 that this
symmetry which had since long been studied in pure mathematics
could be used for the classification of all strongly interacting
particles. Assuming the validity of the new symmetry which
includes the symmetry corresponding to charge-independence,
Gell-Mann found that his earlier multiplets could be brought
together into larger groups called supermultiplets each
containing all baryons or all mesons which have the same spin and
the same parity, i. e. have the same measure for their rotation
around their axes and are transformed in the same way by
reflections. Gell-Mann called this classification "The Eightfold
Way". The nucleons were found to belong to a supermultiplet of
eight particles i.e. an octet. For the mesons an octet was
proposed were the pi- and K-mesons filled seven places.
Because one place was empty a new meson was predicted. Its
existence had been suspected already by some of the Japanese
physicists mentioned above. It was soon discovered which meant
that Gell-Mann's theory was strongly supported. Still more famous
is Gell-Mann's prediction in 1962 of a new baryon called omega
minus.

A similar classification was proposed by Y. Néeman somewhat
later than Gell-Mann.

Gell-Mann has also found that "The Eightfold Way" can be
described very simply by assuming that all particles which
interact strongly with each other are composed of only three
kinds of particles which he called quarks and of the
corresponding antiparticles. The quarks are peculiar in
particular because their charges are fractions of the proton
charge which according to all experience up to now is the
indivisible elementary charge. It has not yet been possible to
find individual quarks although they have been eagerly looked
for. Gell-Mann's idea is none the less of great heuristic
value.

And interesting application of "The Eightfold Way" is the
so-called current algebra which was founded by Gell-Mann. It has
e.g. made evident that there are important connections between
the different kinds of elementary particle interactions.

Gell-Mann has given many fundamental contributions to the theory
of elementary particles besides those which have been mentioned
here. He has during more than a decade been considered as the
leading scientist in this field.

Professor Gell-Mann. You have given
fundamental contributions to our knowledge of mesons and baryons
and their interactions. You have developed new algebraic methods
which have led to a far-reaching classification of these
particles according to their symmetry properties. The methods
introduced by you are among the most powerful tools for further
research in particle physics.

On behalf of the Royal Swedish Academy of Science, I congratulate
you on your successful work and ask you to receive your Nobel
Prize from the hands of His Majesty the King.